1. Field of the Invention
The present invention relates to the production of urea-isocyanurate copolymers which are useful particularly but not exclusively in Reaction Injection Moulding (RIM) applications and Structural Reaction Injection Moulding.
2. Discussion of the Background
Reaction Injection Moulding employs intimate mixing (usually by impingement) of at least two streams of reactive liquids followed by their injection into a mould where polymerization and fabrication occur simultaneously. A development of the RIM technique is Structural Reaction Injection Moulding (SRIM) which is used for the production of composite materials. In SRIM, a reinforcing material (usually a fibrous mat) is pre-placed in a mould so as to be incorporated in the final moulded article. The successful operation of the SRIM technique requires that the reinforcement be adequately wetted by the reactive liquids prior to gelling.
It is well known to produce polyurethane and polyurea articles by RIM. Thus for example polyurea formation may be effected by mixing a liquid stream comprising a polyisocyanate compound (e.g. an MDI derivative) and one or more other liquid streams which comprise polyamino compound having a flexible chain (usually a polyalkyleneoxy chain) connecting the amino groups, and a chain extender which is a comparatively low molecular weight aromatic diamine. Typically the chain extender is used in an amount of at least two equivalents (of chain extender) per equivalent of the polyamino compound (see for example WO-A89/06251).
However, such polyureas have the disadvantage that they have a high mould filling viscosity and a very short gel time and are unsuitable for SRIM processing.
Polyisocyanurate polymers are already known and are obtained by trimerisation of an organic polyisocyanate compound in the presence of a trimerisation catalyst (many examples of which are known) to produce a high cross-link density product which comprises of the `interconnected` isocyanurate rings.
Solid polyisocyanurates generally suffer from brittleness resulting from the high cross link density. Attempts to overcome the brittleness problem and produce polymers which are useful in RIM applications is described in U.S. Pat. No. 4,126,741 and U.S. Pat. No. 4,126,742 which disclose the reaction of the polyisocyanate with up to a total of about 0.4 equivalents of specific polyol combinations (to provide adducts containing urethane linkages) prior to trimerisation of the remaining free isocyanate groups to form the isocyanurate networks. The resulting products are thus urethane isocyanurate copolymers. However, such products are associated with the disadvantage that they have limited stability above 100.degree. C. due to the presence of the PU catalysts required for their preparation.
It has also been proposed in U.S. Ser. No. 247,270 to form urea isocyanurate copolymers which are useful in RIM applications. The copolymers disclosed in U.S. Ser. No. 247,270 are produced, in the RIM process, by the reaction impingement mixing of a polyalkylene oxide amino, an amino terminated chain extender, and an aromatic polyisocyanate as well as a catalyst for effecting isocyanurate formation. The preferred isocyanate index of the mixture is in the range of about 1.2-2.0. The reaction proceeds by initial `capping` of the aminos by the excess isocyanate followed by trimerisation to form the isocyanurate network and thereby produce a urea isocyanurate copolymer. However, a disadvantage of such products is that they have mould filling viscosities which are too high for successful use in SRIM applications.